A Journey To The Oort Cloud, Where Comets Are BornThe comet ISON, discovered by two amateur astronomers last year, will zoom past the Earth next fall. But where did it come from? Astronomer Andrew Fraknoi says a passing star could have flung the comet our way from the Oort Cloud, a distant realm of ice chunks at the outer limits of the solar system.

The comet ISON, discovered by two amateur astronomers last year, will zoom past the Earth next fall. But where did it come from? Astronomer Andrew Fraknoi says a passing star could have flung the comet our way from the Oort Cloud, a distant realm of ice chunks at the outer limits of the solar system.

Back in September, a few amateur astronomers in Russia spotted a little blip of light in the night sky. That dot of light wasn't on any of their star charts, and it was moving. It was not a UFO. They discovered a new comet called ISON. It's zooming towards the sun right now, and it might outshine the full moon when it's scheduled to fly by the Earth near the end of the year.

Oh, you don't want to wait that long? There's comet PANSTARRS - not going to be as big or bright, or scheduled to be. It's scheduled to drop by this March.

Where do they come from? Is ISON a return visitor, and what kicked this dirty snowball toward the sun and in our direction in the first place? Joining me to talk more about it is Andrew Fraknoi, chair of the astronomy department at Foothill College in Los Altos, California. He joins us from NPR West in L.A. Good to talk to you, Andrew. Happy New Year.

ANDREW FRAKNOI: Same to you, Ira. Nice to be back with you.

FLATOW: This ISON sounds like the sight of a lifetime.

FRAKNOI: Well, this is - I want to say, first and foremost, that predicting comets is a lot harder than predicting the stock market.

(LAUGHTER)

FLATOW: And you know how difficult that is.

FRAKNOI: So I don't want us to promise NPR listeners anything like the full moon idea. That actually turns out to be way overestimated, most likely. But it might turn out to be an extremely dramatic comet that's well-placed for observation, kind of a Christmas gift for 2013.

FLATOW: And it would be easy to see?

FRAKNOI: Again, no promises...

(LAUGHTER)

FRAKNOI: ...but yes, it might be. You know, over the years, your older listeners will remember things like comet Kohoutek, and last year we had comet Elenin, both of which were going to be a big deal and turned out to be not such a big deal. So these comets, especially the ones that come from far away, can be very unpredictable in terms of what happens to them as they round the sun and come back around to the outer parts of the solar system. Some evaporate a lot, some fall apart, and so it's - it'll be much easier to talk to you about this next December rather than now.

(LAUGHTER)

FRAKNOI: Still, we want to keep our listeners alert to the possibility that two good comets might be visible in the next year.

FRAKNOI: Sooner, that's right. It will go around the sun, March 10th, about 28 million miles from the sun. It'll make its turn, and then come back around toward the earth again. And for about 10 days after that, if you look in the really early evening sky, it might make a nice comet with a tail visible. Again, we'll know as we get closer to it how good it'll be, but if you want to put something on your social calendar, March 13th, we're estimating that it's going to be just a little bit below the crescent moon and easy to spot.

FLATOW: Wow. OK. I'm putting that down in my calendar to make sure we go out and see that. Where do the comets come from? We've talked about the Oort cloud being way out there, but I don't think people have a concept how far out the Oort cloud is.

FRAKNOI: It's absolutely true. There are two reservoirs, Ira, that comets come from. There's an outer reservoir which you say, correctly, is called the Oort cloud, an enormous round cloud of comets which is 50,000 times further from the sun than the Earth is. There's also an inner region called the Kuiper Belt, or that Kuiper Zone, from which comets can come. But it looks like comet ISON, the one that we're looking at, is coming now from the Oort cloud, is coming from further away. It - and because it is new as a visitor to the inner solar system, that's what we - that's why we don't know how good it will be at evaporating and breaking apart, at making a spectacle of itself.

FLATOW: 1-800-989-8255. Talking with Andrew Fraknoi. Andrew, how do we know that it's a new comet?

FRAKNOI: Well, this is really interesting. What you do with comets is you watch them move across the sky, and from that you trace back where they come from. An old comet like comet Halley, which came around long ago and got trapped by the gravity of our giant planet, that doesn't go very far out. It just comes around and around every 76 years. What we've noticed with comet ISON's orbit is that it's an orbit that takes us way out there. And so because of that, we think it's either a first-time visitor, or it's been a long, long time since it's been around the block, so to speak.

FLATOW: When you say gravity that was trapped by our giant planet, did you mean the sun instead of the planet?

FRAKNOI: No. I mean, Jupiter...

FLATOW: Oh, Jupiter. I see.

FRAKNOI: ...Saturn, Uranus and Neptune all have the ability, if a comet to goes close to them, to pull it in with their gravity and change its orbit forever. And that's what happened with Halley's Comet. Its orbit got changed long ago, and now it's an absolutely regular visitor every 76 years.

FLATOW: Could a far away - really far away item, like a star disturb the orbit of, you know, kick it into our solar system?

FRAKNOI: Right. So that's what we think happens when a new comet comes to visit. Why, after being out there for billions of years, does a chunk of ice suddenly decide, this Thursday, I'm going to make my way in toward the inner solar system? There must be something that disturbs it motion, either a collision with another comet, or more likely something that passes by that giant reservoir way out there - a passing star, a group of objects out there that have an interaction of some kind. Something kicks a new comet inward, and that may well be what happened with comet ISON.

FLATOW: And when we see meteor showers, we're going through the remnants of a tale of a comet that was here before.

FRAKNOI: That's exactly right. So when you see a shower of shooting stars, what's happening is that the Earth, in its yearly motion around the sun, intersects a swarm of - can I put it so unromantically - cosmic dirt leftover from a comet. When Halley's Comet or any of these regular comets come by, when the ice in them evaporates, the dirt gets left behind, and that dirt then is strewn around on the orbit. And if the Earth intersects a swarm of dirt like that, then we get a lot of meteors or shooting stars. And this next year, we think the Perseid meteor shower - which has its best time around the middle of August, August 12th will be the best night - that would be in a specially good time to look for comet dirt in the sky.

FLATOW: What keeps the dirt in the comet all clumped up together like that?

FRAKNOI: It's frozen. It's interesting that a comet is a chunk of dirty ice left over from the time that our solar system - the sun and its family - first formed. And there was a gaseous material out there with frozen ice, and there was actual solid material, pieces of dirt and rock, which froze within the ice. And that frozen deep-freeze out there keeps everything so cold that it - that the dirt never gets loose until the comet gets near the sun. Then when the ice evaporates, the dirt gets lucent, and we can actually see a tail of dirt coming off the comets after they've been around the sun.

FLATOW: So a comet is really like a giant, dirty snowball?

FRAKNOI: Dirty snowball, dirty ice ball is exactly the right description. And as each piece of dirt burns up in the atmosphere, making a beautiful shooting star, what you need to keep in mind is that you are seeing the death of chunk of material that's been around for billions of years in this great cosmic deep-freeze, and is now putting on a show just for you.

FLATOW: Wow. 1-800 - very, very lyrically put, Andrew. 1-800-989-8255. Let's talk about - because I know you love to talk about it, and I love to talk about it - what there is to see in the night sky these days. Are there any treats up there? I know I have been watching Jupiter for the last...

FRAKNOI: Well...

FLATOW: ...number of months, and it's just been gorgeous.

FRAKNOI: That's right. This is going to be a good year for watching Jupiter. And, again, if your listeners want to get a particular date on their calendars, January 21 is going to be an exciting day, because on that night, in the evening sky, the crescent moon will be less than one degree apart from Jupiter. So you could put just about a full moon and a half between Jupiter and the moon.

So even if you don't know in general which dot in the sky, which bright dot happens to be Jupiter, and you don't have an astronomer handy to ask - if you go out on January 21, you'll be able to spot the crescent moon easily enough in the evening sky, and then just look very close to it, there'll be a bright dot of light.

Also this year, Saturn will go into opposition, April 28th. And that, when it go - when a planet goes into opposition, it means it's opposite to sun in our sky. So when the sun sets, the planet rises. It does the opposite of the sun. So starting in April, you'll be able to see Saturn in the sky all night long. And if you've got a small telescope or you can go to your local observatory or planetarium or astronomy department, they can show you the beautiful rings of Saturn all night long.

FLATOW: Hmm. And it will still be cool enough, drier air - or we hope that the rains don't come in April to cloud up the sky to see it, but I love it when that opposition happens and you can see all that stuff. 1-800-989-8255. Lots of people would like to talk to you. Let's to go another Jeremy, this one is Jacksonville, North Carolina. Hi, Jeremy.

JEREMY: Hi, Ira. Thanks for taking my call.

FLATOW: Mm-hmm. Go ahead.

JEREMY: I had two quick questions. One: What's the difference between, you know, the asteroids that are constantly fating the planet in movies and comets? And also, what - how big are these comets that they leave perpetual meteor showers? And I'll take my answers off the air.

FLATOW: All right. Thanks for calling. Good questions.

FRAKNOI: Yes. Good questions. So, this is interesting. The difference between comets and asteroids has to do with, again, the formation of our solar system long ago. There was a lot of junk left over. Janitorial services were not up to snuff. Nobody cleaned up. And so we have a lot of material left over. The rocky material left over is called asteroids. The icy material is called comets. And asteroids generally hang out in a belt between Mars and Jupiter, but some of them can hang out much closer to the Earth. And it's those Earth-crossing asteroids that you hear about in Hollywood disaster movies.

And we are actually tracking Earth-crossing asteroids very carefully to make sure that none of them have our name on it, so to speak. But that's the difference between asteroids and comets. Now, as far as the size of these objects, they can vary tremendously. You can have icy chunks as small as a few feet across, or rocky chunks as small as a few across or even smaller - but we don't treat them with respect if they're much smaller than that - all the way to many, many miles across.

Halley's Comet, we estimate, is about eight miles across, a giant chunk of ice. And so there's a tremendous range in these. The ones that we worry about, of course, are the big ones. And the ones that put on a better show are the big ones, as well.

FLATOW: Talking with Andrew Fraknoi on SCIENCE FRIDAY, from NPR. I'm Ira Flatow. Well, let's go to the phones. Lots - boy, people love to talk about this kind of stuff. I'm glad they do. Ed in San Francisco. Hi, Ed.

ED: Hi. Good afternoon. I'd like to ask Andy to mention - I look at the ugly part of this object, whether they're comets or asteroids, who really are in big, big surprise. And very often, we don't even see some of them until they're leaving our view. So we don't see them. Very often, we don't see them coming in. I wanted Andy to mention something about Apophis, which is supposed to come real close to Earth, (unintelligible) Earth in the year 2029, Friday the 13th, April.

It also is going to come back again, Friday the 13th again. I don't know what month it is, but it's going to be 2036. And it has much higher chance of zapping the Earth with a collision. It's about 750 feet, so a pretty good sized asteroid.

FLATOW: All right. Let me get a...

ED: If you could just have him mention - comment about that.

FLATOW: OK. Andrew, do you know what's with that?

FRAKNOI: All right, yes. There is an asteroid called Apophis, which is named after the Egyptian god of darkness. And it is one of those that crosses the Earth's orbit. It is going to be coming by, actually - you're going to laugh - on Friday, April 13th, so Friday the 13th. But it will be...

FLATOW: We'll cover it live.

(LAUGHTER)

FRAKNOI: Oh, there you go. And it'll be a safe distance away. But there was some concern that on a future path - because this is a regular returning comet - that on a future path, Apophis may actually collide with the Earth. That was an early observation. Today, we don't think so, although each time such an asteroid comes by, our gravity changes its orbit a little bit. So we're going to be monitoring all of these Earth-crossing asteroids very carefully in the future to make sure that they're safe for us.

FLATOW: A tweet came in from Todd Mills(ph), who says: How much weight does the Earth gain from shooting stars and such each year?

FRAKNOI: This is a question my students ask all the time, is because we do get a lot of material hitting the Earth's atmosphere - most of it, thank goodness, burning up, because our atmosphere is so thick and so good at burning things up. And so they worry that maybe this impact of these materials either changes the Earth's orbit or, you know, gives the Earth extra weight, so it has to go to Jenny Craig to lose weight.

And the good news is that we live on an enormously large planet. So the amount of material that hits us each year is absolutely insignificant compared to the mass of our enormous ball of rock. And so there isn't any change as a result of that.

FLATOW: Let's go to Mark in South Bend, Indiana. Hi, Mark.

MARK: Hi, Ira.

FLATOW: Hi, there.

MARK: How are you guys?

FLATOW: Fine. How are you? Go ahead.

MARK: Fine, thank you. I was wondering what gives the comets and the asteroids their thrust to travel (unintelligible).

FLATOW: Yeah. How fast are they moving, Andrew?

FRAKNOI: Yeah. Well, that depends a lot on where they are. But it's an excellent question. Why are things in space moving at all, right? You could ask the same question about the Earth. If the Earth weren't moving, it would long ago have fallen into the sun. And we think that there were probably planets and asteroids and comets early in the history of the solar system that didn't have enough speed and fell into the sun and made the sun a bigger star than it was at the beginning.

But the objects that are left today are the ones that haven't fallen into the sun, that were born from the swirling cloud that gave rise to the solar system five billion years ago and had enough motion to orbit rather to fall in. So it was the original swirling motion of the mother cloud that gave comets, asteroids and everything else in the solar system the speed they need to avoid the gravity pull of the sun.

FLATOW: And there's the topic to think about this evening. Why is everything moving? Where is it going to, how it got started and why is it heading in our direction? Thank you, Andrew. As always, a terrific job. Thanks a lot and...

FRAKNOI: Oh, always fun to be on, and you have great questions.

FLATOW: And we'll be looking January 21st for this first event coming to see for us in the night sky. Andrew Fraknoi is chair of the Astronomy Department at Foothill College in Los Altos, California.

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